This invention relates to discrete multitone transmission (DMT) of data by digital subscriber loop (DSL) modems and more specifically to the allocation of bits, respectively, to the discrete multitones.
In digital communication systems employing multi-channel or multi-carrier transmission, the most effective allocation of bits to the channels has been discussed in the literature. The well-known solution from information theory, analogized to pouring water over a terrain defined by the noise/attenuation of the channel transform characteristic, has been found to insure efficient use of signal power within limits defined by aggregate power and power spectral density mask limits. However, the method in some instances may not go as far as possible in exploiting available power imposed by these limits.
For heuristic purposes, the prior art and the invention are discussed in terms of N quadrature amplitude modulation (QAM) channels with a uniform symbol rate and a non-uniform (unique to each channel) QAM constellation. QAM, a form of combined amplitude and phase modulation, represents k-bit sets of data by modulating two (orthogonal) quadrature carriers, cos27xcfx80ƒxcex6 and sin2xcfx80ƒxcex6 to generate a pulse whose phase and amplitude convey the encoded k-bits of information. The QAM signal tone can be viewed as a phasor in the complex plane, each distinguishable phasor representing a unique state of the tone identified with one unique value in a range. Thus, if the channel and signal power are such that 4 separate phasors can be reliably distinguished, the scheme allows two bits to be represented For 3 bits to be represented, 8 phasors must be distinguished and so on. The number of different phasors or states that are distinguishable in a single tone (pulse), the QAM constellation, is limited by the signal to noise ratio of the channel and limits imposed by external standards as discussed below.
In a DMT modem a transmission frequency band is separated into N sub-bands or frequency bins, each corresponding to one QAM channel. In a non-ideal channel each sub-band has a different capacity as a result of the variation of noise and attenuation with frequency. In addition, external standards impose limits on the aggregate power of a signal (the power applied in all sub-band channels) and a cap on the power as a function of frequency defined by a power spectral density mask.
The power spectral density mask may be dictated by the standard used in a particular country implementing the standard (such as A.N.S.I. standard T1.413-1995). The mask may also be a design constraint intentionally imposed by a modem designer for some other reason. For example, a designer may intentionally impose a constraint that no more than n bits are to be transmitted on transmit channel frequency. Similarly, the designer may impose a constraint that a minimum of bits (or no bits) must be transmitted on a particular tone or frequency. For example, the power limit for frequencies or tones between 0 and 200 kilohertz must be less than xe2x88x9240 dBm/Hz (a power level referenced to one milliwatt over 1 Hz bandwidth). Above 200 kHz (to frequencies in the megahertz of spectrum), the constraint may be xe2x88x9234 dBm/Hz.
Referring to FIG. 1, the attenuation+noise characteristics of a medium can be graphically represented by a floor in a power spectral graph. The lower curve, the channel transform characteristic A represents this floor, that is, the combined effect of noise and attenuation as a function of frequency. A certain margin of power is required to meet or exceed the minimum threshold of a signal for reliable data transmission. In other words, the power of a signal in a given sub-band must be sufficiently high to carry a minimal (1-bit) QAM tone to obtain a predefined bit error rate. The minimum margin, that required to transmit a single bit, is represented by curve B. Curve C represents the limits imposed by a power spectral density mask imposed by an eternal communications standard. The other limit is on the aggregate power, also defined by an eternal communication standard, e.g., ANSI Standard T1.413-1995 limits the total power for all sub-bands to 100 mWatts. Some coding techniques, such as Wei code described in American National Standard for Telecommunicationsxe2x80x94Network and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Line Metallic Interface, ANSI T1.413-1995, may also require a minimum number of bits in a frequency band if the band is to convey any information at all. In other words, if the power spectral density mask limit may require that less energy be used than the minimum required to transmit a single bit.
Note that the minimum allowable size of the power margin is, in part, arbitrary since, to an extent, it is defined in terms of some a priori rules and technical criteria (which are arbitrary to the extent that they establish a dividing line between acceptable and unacceptable error rates; Bit Error Rate or BER) for the given communication system. Note also that the size of the margin available for a given sub-band corresponds to the dimension of the constellation that can be represented in a signal carried in that QAM channel. That is, the larger the margin in a band, the greater the number of states that can be reliably distinguished in that band and the larger the constellation that can be used.
The above context creates a bit-allocation problem That is, given the constraints, how should bits be allocated among the available QAM channels to provide the highest possible data rates? DSL modems that use DMT modulation concentrate the transmitted information in the frequency sub-bands that have the minimum attenuation and noise. The optimum distribution of transmission power is obtained by distributing the power according to the well-known xe2x80x9cwater pouringxe2x80x9d analogy as described in Robert G. Gallagher, Information Theory and Reliable Communication, John Wiley and Sons, New York, 1968. Such optimal distribution requires a strategy for allocating bits to the sub-bands for the idealized situation where the channel sub-bands approach zero width (xcex94ƒxe2x86x920). For discrete bits, the applicable metaphor could be described as an ice-cube pouring analogy.
DSL technology was conceived to maximize the throughput on twisted pair copper wiring with attendant background noise, time-variant Far End Cross Talk (FEXT) and Near End Cross Talk (NEXT). To determine the transform characteristic of the channel, the modems negotiate during an initial channel signal-to-noise ratio (SNR) estimation procedure. During the procedure, the transmitter sends a known pseudo noise (PN) signal. The receiver computes the characteristics of the received signal in the form of a ratio Nk/gk, where gk is the channel gain (inverse of the attenuation) in frequency band k and Nk is the noise power in the band k. The literature contains many algorithms for determining the power distribution across the full frequency bandwidth for maximum data throughput. As noted above, the optimum approach for non-uniform Gaussian noise channel divided such that each band can be considered an additive white Gaussian noise channel has been proved to be the xe2x80x9cwater pouringxe2x80x9d algorithm of power distribution. In this case, the gk/Nk Profile is compared to a terrain and the available aggregate power limit to a fixed supply of water poured over the terrain. The depth of the water corresponds to the power spectral density. The water pouring analogy is inappropriate to allocation of power in digital channels intended for transmission of binary data (bits).
According to one method of allocating bits (John A. C. Bingham, Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come, IEEE Communications Magazine, May 1990, pp5-14), frequency sub-bands or bins are xe2x80x9cfilledxe2x80x9d with data bits one bit at a time. A bit is added to the bin for which the marginal power cost is the lowest. That is, a bit is added to the bin such that transmission in that bin is the least expensive, relative to an additional bit in any other bin, in terms of power needed for the resulting signal constellation to be received at a predefined BER. The filling procedure is followed until the total Power Budget is used up. Since power can only be allocated in discrete amounts corresponding to each bit, the procedure is likened, as mentioned, to an ice-cube filling procedure rather than a water-filling procedure.
It is an object of the invention to provide a method for transmission in a multitone communication system together with an algorithm for allocating bits in the system.
It is an object of the invention to provide a method for transmission in a multitone communication system subject to an aggregate signal power constraint together with an algorithm for allocating bits in the system.
It is an object of the invention to provide a method for transmission in a multitone communication system subject to a signal-power spectral density mask constraint together with an algorithm for allocating bits in the system.
It is an object of the invention to provide a method for transmission in a multitone communication system subject to an aggregate signal power constraint and a signal-power spectral density mask constraint together with an algorithm for allocating bits in the system.
It is another object of the invention to provide a method for transmitting data over multiple interfering channels.
It is another object of the invention to provide a method for transmitting data over multiple interfering channels and a method for reducing interference between the interfering channels.
It is another object of the invention to reduce near end cross talk between DSL modems communicating over the same cable.
Briefly, high transmission capacity in a twisted pair signal line, where power is limited by a power spectral-density mask and an aggregate signal power constraint, is obtained by: (1) allocating data to multitone sub-bands according to a lowest marginal power-cost per bit scheme and (2) in an environment where an aggregate power budget remains after all bits have been allocated to all sub-bands with sufficient margins to carry at least one bit, assigning additional bits to sub-bands with otherwise insufficient power margins to carry a single bit, by frequency-domain-spreading a single bit across several sub-bands at correspondingly reduced power levels, to permit the otherwise unacceptable noise levels to be reduced on average by despreading at the receiving end. In an environment in which multiple interfering channels are employed, signal throughput is increased by (3) forming a number of sub-bands for spreading blocks of data that is equal to a number of interfering channels and multiplying the signal carried by corresponding sub-bands in the separate interfering channels by a different respective vector from an orthonormal basis set so that near-end cross-talk is eliminated upon despreading at the receiving end.
Note that xe2x80x9cspreadingxe2x80x9d as used in the present application, refers to a process applied at a stage where the signal is decomposed into spectral elements, so that it can be applied selectively to frequency components, in contrast to conventional spreading found in, for example, wireless (cellular) telephony, where spreading is applied to the signal time series, and affects (spreads) all elements of the spectrum equally as a consequence.
According to the invention bit allocation may be performed to optimize throughput within aggregate power and power spectral density mask limits. Some method, such as the approach identified above with the water pouring analogy, may be used for this bit allocation. The process of bit allocation will be limited either by the mask limit or the aggregate signal power limit. If after efficient allocation, the total signal power is less than the aggregate power limit, there will usually be unused sub-bands. These unused sub-bands were rejected in the initial bit-allocation process because the available power margin in them was insufficient to transmit a single bit. That is, the channels were identified as unusable because transmitting a single bit was found to exceed the mask limit for the channel. In this case, where the bit allocation process is limited by the mask, the channels with low power margins are used to transmit information by spreading a single block of data (one or more bits) over multiple channels and then despreading them at the receiver.
The device of spreading and despreading over multiple channels also provides a mechanism for reducing near end cross talk (NEXT). The context to which this device applies is a packet consisting of I interfering channels and nI carriers in each channel. For example, the channels could be four wire pairs, in each, some multiple of four carriers are used to convey information by spreading a single block over each of four carriers to transmit, and then despreading at the receiver. At the transmitter, however, the signals in each interfering channel are multiplied by one element of an I-dimensional orthogonal code (such as a binary code). At the receiving end, the signals are multiplied again by the respective opposite orthogonal code and then despread. The process of despreading not only reduces incoherent noise as in the embodiment discussed above, but it also substantially eliminates NEXT because the interference generated in all the frequency channels, being derived from orthogonal set, cancel each other. Thus in a channel of four twisted pairs of wires, each pair transmits a different block of data but every different block is spread over four carriers in a given wire pair. The signal transmitted over each of the four wire pairs is assigned one of four orthogonal codes. Summing each block spread over the four frequency channels causes mutual cancellation of the four induced cross-talk signals of the four wires that were multiplied by the four orthogonal codes.
Discrete Multitone (DMT) modulation serves as a framework to demonstrate the spreading process. An input data stream is segmented into small blocks of bits, and each such block is re-expressed as a complex number. For example, a constellation of 16 possible discrete complex number values can be used to convey 4 bits, since 16 different states are required to represent 4 bits. The resultant array of complex numbers is inverse-Fourier transformed to synthesize a time series, Y(t), that represents a sum of multiple distinct sinusoids. (A complex conjugate array of complex numbers is used as an input to the Inverse Fast Fourier Transform process to assure a real resultant time series.)
Each of the complex numbers used to encode data therefore plays the role of a complex spectral coefficient. That is, each defines the amplitude and phase of one of the orthogonal sinusoids included in the transmitted waveform. The number of discrete points in the constellation for each of the bands is a consequence of the measured attenuation and noise level in that frequency band, based on a bit-allocation process that need not be described here.
In both of the above schemes, the signal power in each frequency carrier is reduced in proportion to the number of carriers used. Also, in both schemes, the information relating to the number of bits per block the frequency channels over which blocks are to be spread, etc. must be shared between the transmitter and the receiver. Regarding the latter scheme, the transmitter and receiver must also share the orthogonal codes to be used for each twisted pair, though these can be established on a permanent basis.
According an embodiment, the invention provides a transmitting modem that receives digital data from a data source and modulates separate carriers to represent the digital data. The modulated signal is applied to a channel connected to a receiving modem. The channel is subject to a power spectral density mask. The transmitting modem includes first, second, and third signal modulators, each with an input. The modem also has a signal combiner with a combined output connected to the channel and a serial-to-parallel converter connected to the data source and to each of the first, second, and third signal modulator inputs. The connection is such that the digital data from the data source is converted to multiple parallel streams applied respectively to the first, second, and third signal modulators. Each of the first, second, and third signal modulators has a respective output connected to the signal combiner such that a sum of output signals of the first, second, and third signal modulators is applied to the channel. A transfer characteristic of the channel is such that a first minimum power required to represent a specified minimum number of bits by modulating in a first frequency sub-band falls below the power spectral density mask and such a that a second minimum power required to represent a second specified minimum number of bits by modulating in each of second and third frequency sub-bands exceeds the power spectral density mask. The serial-to-parallel converter is programmed to feed a first bit of the digital data to the first signal modulator to represent the first bit by modulating in the first frequency sub-band at a first power level at least as high as the first minimum power. The serial-to-parallel converter is also programmed to feed a second bit of the digital data to the second and third modulators to represent the second bit by coherently modulating in both the second and the third frequency sub-bands at a second power level below the first power level, whereby resulting signals applied in the second and third frequency sub-bands may be combined by the receiving modem to retrieve the second bit. The first and second minimum number of bits are both equal to one in the absence of some other specified constraint.
According another embodiment, the invention provides a frequency division multiplexor transmitting data from a data source over a channel. The multiplexor has a signal modulator with an input and first, second, and third outputs, each output transmitting data in a respective one of first, second, and third frequency bands. A channel response detector connected to the channel detects a transfer characteristic of the channel, the transfer characteristic including a noise power level and an attenuation of the channel. A controller connected to the signal modulator controls an allocation of first and second blocks of data from the data source for transmission in the first, second, and third frequency bands. The controller is programmed to transmit the first block of data in the first frequency band and transmit the second block redundantly in each of the second and third frequency bands at a first power level when the channel transfer characteristic is such that a power level required to transmit the second block, at a specified bit error rate, in the second frequency band alone is a first power level. However, when the channel transfer characteristic is such that a power level required to transmit the second block, at the specified bit error rate, in the second frequency band alone at a second power level, the second power level being higher than the first power level, the controller transmits the second block in the second frequency band alone.
According still another embodiment, the invention provides a modem with a frequency-division modulator and a controller. The modulator transmits input data in separate frequency channels. The controller has a memory that stores a power spectral density (PSD) mask specifying the maximum power levels permitted for each of the frequency channels. The controller""s memory also stores an aggregate power limit specifying a total permitted power for all of the signals in all of the channels. The controller is programmed to measure and store in the memory the channel transfer characteristic of a communications channel through which the input data is to be transmitted. The controller is also programmed to transmit respective unique portions of the input data in of the frequency channels based on the stored aggregate power limit, the PSD mask, when the measured transfer characteristic is a first transfer characteristic. The controller is programmed to transmit a same portion of the data in at least two of the frequency channels responsively to the stored aggregate power limit, the PSD mask, when the measured transfer characteristic is a different transfer characteristic.
According still another embodiment, the invention provides a method for use in a data modulator for allocating bits to data channel frequencies. The method includes the following steps: (1) storing mask power data representing a respective maximum power level for each of the data channel frequencies; (2) storing aggregate power data representing a total amount of signal power to be applied in all of the channel frequencies; (3) allocating bits on a per frequency basis, such that bits are successively allocated until the respective maximum power level is at least substantially reached for each of the channel frequencies and such that each of the bits is allocated to a single respective one of the channel frequencies; and (4) when the aggregate power level is not substantially reached in the step of allocating, further allocating bits to multiples of the channel frequencies for transmission at reduced power rates per channel frequency, to permit further bits to be allocated, until one of the aggregate power limit is substantially reached and the respective maximum power level is reached for each of the data channel frequencies.
According still another embodiment, the invention provides an apparatus that allocates bits for data transmission via a multiple discrete frequencies. The apparatus has tone ordering circuitry, gain scaling circuitry and an inverse discrete Fourier transform modulator. The circuitry in combination allocates initial bits to frequencies on a per frequency basis, such that the initial bits are successively allocated until a maximum power level for each frequency is at least substantially reached, each of the initial bits being unique to a given frequency. The circuitry also calculates a stored total power level for the initial bits allocated to a plurality of transmit frequencies, and if the stored total power level is not exceeded, allocate further bits to frequencies for which no initial bits are allocated, such that each of the further bits is redundantly allocated to more than one of the frequencies.
According another embodiment, the invention provides a frequency-division multiplex (FDM) transmission system for a channel having multiple subchannels, each of the subchannels being susceptible to cross-talk interference from another of the subchannels. The system comprises a transmitting modem with a programmable FDM modulator connected to modulate first and second frequency carriers, representing an input data stream, in each of first and second subchannels of the channel. Also, the system includes a receiving modem connected to the channel and a modulator programmed to modulate the first and second frequency carriers coherently to represent a first subportion of the data stream in the first and second frequency bands to form first and second signals in the first subchannel. The modulator is programmed to modulate the first and second frequency carriers coherently to represent a second subportion of the data stream in the first and second frequency bands to form third and fourth signals in the second subchannel. The receiving modem has a demodulator configured to combine coherently the first and second signals. The modulator is also programmed to form the third and fourth signals such that when the demodulator combines coherently the first and second signals, cross-talk interference in the first subchannel, caused by concurrent transmission of the third and fourth signals in the second channel, is diminished in a combined signal resulting therefrom.
According to still another embodiment, the invention provides a method for reducing near end cross talk. The method performs the following steps: (1) forming first and second signals in respective first and second tones redundantly representing first data to form a first multi-tone signal such that the first and second signals are weighted by a first vector of an orthogonal set of codes; (2) applying the first multi-tone signal to a first interfering channel; (3) forming third and fourth signals in the respective first and second tones redundantly representing second data to form a second multi-tone signal such that the third and fourth signals are weighted by a second vector of an orthogonal set of codes; (4) applying the second multi-tone signal to a second interfering channel; and (5) combining the first and second multitone signals such that a distortion in the first data caused by near end cross talk in first interfering channel is diminished.
According to still another embodiment, the invention provides a frequency-division multiplex (FDM) transmission system for transmitting an input data stream through a channel. The system has a transmitting modem with a programmable FDM modulator connected to modulate first and second frequency carriers, representing an input data stream, in the channel. The system also uses a receiving modem connected to the channel. A modulator is programmed to modulate the first and second frequency carriers coherently to represent a first subportion of the data stream in the first and second frequency bands to form first and second signals. The receiving modem has a demodulator configured to combine coherently the first and second signals to extract the first subportion such that an incoherent distortion of the first and second symbols in the first channel is, on average, reduced in the extracted first subportion.
According to still another embodiment, the invention provides a method for increasing a data rate in a communication channel subject to a power spectral density mask. The method follows these steps: (1) detecting a transfer characteristic indicating a required minimum power of a respective carrier modulated to transmit one bit in each of a plurality of multitone subchannels of the channel; (2) supplying a data stream to a modulator; (3) modulating a first set of respective carriers to represent respective unique portions of the data stream in at least a subset of those of the multitone subchannels for which, in the step of detecting indicates the minimum power falls below a power limit imposed by the power spectral density mask; (4) modulating a second set of respective carriers to represent redundantly at least one portion of the data stream in at least a subset of those of the multitone subchannels for which the step of detecting indicates the minimum power exceeds a power limit imposed by the power spectral density mask; (5) receiving the at least first and second symbols at a receiving end of the communication channel; (6) combining the at least first and second symbols received at the receiving end in such a way as to increase a signal power of the at least first and second symbols and, on average, reduce incoherent distortion of the at least first and second symbols; (7) reconstructing the same portion of the data stream at the receiving end from a combination of the at least first and second symbols, resulting from the step of combining.
According still another embodiment, the invention provides a communications system for communicating data in a channel having multiple subchannels. The system has a modulator with an output for each of the subchannels. It also employs a demodulator with an input for each of the subchannels. The modulator has an input connected to receive data from a data source. The modulator is programmed to modulate separate sets of carriers in each of the subchannels to represent respective portions of the data The modulator is also programmed to modulate n separate frequency (resulting from a modulation of the modulator being output by the modulator) carriers coherently in each of the subchannels to represent a one of the respective portions of the data. Each of the n modulated signals, upon being received at the demodulator, includes an incoherent component resulting from attenuation and/or noise in the channel, a first coherent component resulting from near-end cross talk from the n modulated signals in the subchannels other than the each, and a second coherent component output which is the original n modulated signals output by the modulator. The modulator modulates the n separate frequency carriers such that when the demodulator demodulates the n modulated signals, upon being received at the demodulator, by linearly combining the received n modulated signals in a signal resulting from the linearly combining, the incoherent component and the first coherent component are, on average, suppressed and the second coherent component is, on average, amplified.
According still another embodiment, the invention provides a transmitting modem receiving digital data from a data source, modulating carriers to represent the digital data, and applying a resulting modulated signal to a channel connectable to a receiving modem. The transmitting modem has first, second, and third signal modulators, each with an input. The modem also has a signal combiner with a combined output connected to the channel and a serial-to-parallel converter connected to the data source and to each of the fist and second signal modulator inputs such that the digital data from the data source is converted to multiple parallel streams applied respectively to the first and second signal modulators. Each of the first and second signal modulators has a respective output connected to the signal combiner such that a sum of output signals of the first and second signal modulators is applied to the channel. The serial-to-parallel converter is programmed to feed a bit of the digital data to the first and second modulators to represent the second bit by coherently modulating in both the first and second frequency sub-bands, whereby resulting signals applied in the first and second frequency sub-bands may be coherently linearly combined by the receiving modem to retrieve the bit and such that incoherent components and coherent but at least partially orthogonal components of the resulting signals are attenuated and coherent component of a modulated signal applied by the first and second modulators is amplified.
According to still another embodiment, the invention provides a method for transmitting data through a channel subject to a power spectral density mask limit and an aggregate power constraint. According to the method, a frequency-dependent transmission characteristic of the channel is detected. First and second sub-bands of an aggregate transmission band are defined responsively to a result of the step of detecting, the aggregate power constraint, and the power spectral density mask limit. A first modulated signal is generated, the signal having a first power dynamic range permitted by the power spectral density mask limit, the first modulated signal representing the first portion of the data. A second modulated signal is generated, the second signal having a second power dynamic range permitted by the power spectral density mask limit, the second modulated signal representing the second portion of the data